The invention concerns a method for calibration of a vectorial network analyzer having n measurement ports and m measurement sites (n>1) by consecutive measurement of reflection and transmission parameters at different two-port calibration standards connected between the measurement ports in any sequence, all of which must have a transmission path, and three different n-port calibration standards connected between the measurement ports in any sequence that must have no transmission.
Vectorial network analyzers (VNA) are used for precise measurement of electronic components, as well as active and passive high-frequency circuits and high-frequency assemblies up to antennas.
The form of description of electrical behavior of electronic components, common in high-frequency technology, occurs via their scatter parameters (also S-parameters). They do not link the currents and voltages to each other, but wave quantities. This representation is particularly adapted to the physical circumstances. The so-called scatter parameters of n-ports (n=1, 2, . . . ) are detected, which are optionally converted to 2n-pole parameters (for example, Z- or Y-parameters).
For the waves a1 and a2 reaching, for example, a two-port and the waves b1 and b2 propagating accordingly in the opposite direction, the following relation applies:
            (                                                  b              1                                                                          b              2                                          )        =                            (                                                                      S                  11                                                                              S                  12                                                                                                      S                  21                                                                              S                  22                                                              )                          ︸                      =                          [              S              ]                                          ⁢              (                                                            a                1                                                                                        a                2                                                    )              ,in which [S] is the scattering matrix, which characterizes the electronic properties of the two-port.
A so-called system error correction ensures that precise measurements of the scatter parameters of the components can be carried out with vectorial network analyzers at all. This system error correction requires precise measurement of standards whose electronic behavior is known or can be determined in the context of system error correction.
It is known for this purpose to measure the reflection and/or transmission behavior of unknown or partially or fully known calibration standards within the so-called calibration range at several measurement sites to be optimized with respect to position and number.
From the measured values of the calibration standard, correction data are obtained via special calculation methods, so-called error quantities or coefficients. With these correction data and a corresponding correction calculation, measured values are obtained for each arbitrary measured object that is freed of the system errors of the vectorial network analyzer and the feed lines, for example, from couplings (crosstalk) or mismatches (reflections).
A known calibration method for a two-port model with 10 or 12 error quantities is the so-called ten-term or 12-term method. In the American literature, it is also referred to as SOLT (S: short, O: open, L: load=match, T: thru) and in Europe as TMSO. It is the only system calibration method for two-port network analyzers with only three measurement sites, one measurement site on the measurement channel common for both ports in front of the switch, each of which switches one of the ports for measurement, and another measurement site on the measurement channel of each port. In this arrangement of measurement sites, however, the switch is integrated in the measurement of the calibration standards.
In this TMSO calibration method, used most often in practice, the two measurement ports must initially be connected to determine the correction data, which corresponds to the calibration standard T (T=thru). Three known one-ports, for example, the calibration standards match (M), short (S) and open (O), must then be contacted at each measurement port and measured.
The multiport measurement problem consists of the fact that all measurement ports are coupled to each other via the measurement object. Consequently, a dimension of the outgoing wave at one measurement site, a dimension for the reflected wave at the next site, and finally a dimension for the transmitted wave at another site, which is independent of the terminations of the multiport, are no longer obtained, but one must additionally consider the reflection properties of the other measurement ports in the model.
For this multiport measurement problem, in recent years some solutions have been published and patented. The solution to the multiport measurement problem of Ferrero, described in Ferrero, Pisani, Kerwin, “A New Implementation of a Multiport Automatic Network Analyzer,” IEEE Trans. Microwave Theory Techn., Vol. 40, November 1992, pages 2078-2085, requires a network analyzer with 2n measurement sites with the same demand for calibration standards as the TMSO method. As a result, the requirements on the hardware of the calibration standards are very demanding. In addition, in the method of Ferrero, all calibration standards must be fully known, which is a particular drawback, since completely known standards cannot be perfectly achieved. In addition, the Ferrero method is exclusively based on the seven-term principle, which results in significant measurement errors, because of the inadequate achievability of fully known standards, and because of the sensitivity of the seven-term principle to such model errors, as explained in Heuermann, “Reliable Methods for Calibration of Network Analyzers for Coaxial and Planar Line Systems,” dissertation, Institute of High-Frequency Technology, Ruhr University Bochum, 1995, ISBN 3-8265-1495-5.
A ten-term method is described in DE 199 18 697 A1, which, like the TMSO method, requires only n+1 measurement sites, but exclusively known calibration standards.
The multiport seven-term method, as described in DE 199 18 960 A1, based on an adaptation of the known two-port method to a multiport method, therefore includes the methods TAN, TNA, LAN, TRL, TLR, LLR, LRL, TAR, TMR, TRM, TMS, LMS, TMO, LMO, UMSO, TMN, LNN, TZU, TZY, TYU, LZY, ZZU, YYU, QSOLT and generally require n−1+2 calibration measurements.
Another method of the ATN Company is described in the American patent U.S. Pat. No. 5,578,932. This patent describes, in detail, a so-called test set, with which a two-port network analyzer can be expanded to n ports. In addition, a special calibration device is described, which is required for automatic calibration of this test set.
The calibration device contains, in addition to the standards open, short and match (also termination), an arrangement of different transmission lines that can be connected between the terminals of the calibration device via semiconductor switches. All standards must therefore be fully known, as in the TMSO method. In contrast to the statement in the abstract, however, complete multiport calibration and error correction does not occur.
Instead of this, only two-port paths are calibrated; the remaining ports are not considered (column 18, line 57). In later measurement operation, two-port measurements are carried out in succession. The measurement ports not included in the calibration are then terminated by different reflection standards incorporated within the test sets. For each value of the reflection standard, a two-port measurement is precisely carried out (column 21, line 1). After measurements have been performed on all measurement ports, a result corrected by the systematic error can be calculated from the obtained measured values and the known values of the reflection standards. For measurement of a three-port test object, according to the patent, two two-port measurements from port 1 to port 2 and from port 1 to port 3 are necessary (column 21, line 1 and line 45), in which, for complete characterization of all parameters, the third port of the test object not included must be terminated during measurement from port 1 to port 2 by at least three different reflection standards (column 21, line 28). This means that, for complete characterization of a three-port 3+1=4, two-port measurements are necessary.
A so-called RRMT calibration method is described in DE 10 204 020 037 A1, in which, in contrast to the method just mentioned, not all calibration standards need be known. From transmission standards known with respect to length and attenuation, the reflection behavior of n known impedances, which are achieved on the one-ports, but can be different in comparison with each other, and from n unknown, strongly reflecting standards open and short, the scatter parameters of the unknown calibration standards open and short are initially determined by calibration, in order to determine the error coefficients of the network analyzer with the ten known terms.
However, a problem in each case is that the measurement of electronic components in the wafer structure (on-wafer measurements) is subject to special boundary conditions, especially with respect to attainability of the calibration standards.
In the semiconductor field, it is not uncommon for the user to implement the calibration standards on the wafers themselves. The geometric reproducibility and equality of calibration standards produced in this way is very high. It is then also advantageous that the calibration standards are situated on the same substrate support (semiconductor) as the measurement objects. In addition to the advantages of short travel paths, parasitic elements, as well as transitional effects from the measurement tips to the wafer, can also be “calibrated out.” However, the electronic properties are only achieved in good approximation. In particular, the reflection standard open cannot be produced with the necessary quality.
The reflection standards (R) can be described very precisely on semiconductors, but generally vary sharply with respect to DC resistance values. In the described methods according to the prior art, it is necessary that R-standards be connected on each measurement port with the most identical possible reflection behavior. If this cannot be guaranteed, as is the case in multiport-on-wafer measurements, since standards are regularly arranged at 90° angles to each other, so-called strains occur, which are regularly the source of very large measurement errors.
Another problem is the number of measurement sites, which must be optimized, in particular in automatic measurements, in order to save equipment demands and costs and achieve reproducible measurement results. Here, the deficient attainability of calibration standards with known properties on the wafer level stand in contrast to the requirement for minimization of measurement sites. On the one hand, the use of unknown standards requires the application of the seven-term method in order to determine the scatter parameters of these standards. On the other hand, the seven-term method, because of the aforementioned vulnerability to errors, is suitable for use of only n+1 measurement sites, in which, on each feed line to a port, an additional measurement site is arranged in front of the switch for switching of the corresponding port.